In today's mobile networks, mobile terminals have the possibility to use access points of different technologies, like 3G/4G (3rd resp. 4th generation networks), or LTE resp. WCDMA/UMTS as defined by 3GPP/ETSI standards, WiFi as defined by IEEE 802.11, and potentially other access points. Generally spoken, a mobile terminal may use access points having different capabilities, for example with respect to support of Quality of Service (QoS) behaviour, type and number of bearers etc.
Nevertheless, it is desired that services can be used as seamlessly as possible even when a mobile terminal changes its access point during usage of a certain service or during an active session.
One specific example is the usage of VoLTE (Voice over LTE) as described in GSMA PRDs (Permanent Reference Documents) IR.92 and IR.94, and employing the VoLTE roaming architecture as described in GSMA PRD IR.65. VoLTE is based on the IMS (IP Multimedia Subsystem) architecture and thus uses corresponding network nodes and their capabilities, which are at the same time used by Rich Communication Services (RCS) as described in GSMA PRD RCC.07.
FIG. 1 shows an example of a Packet Data Network (PDN) connection between a mobile terminal 10 and a PDN Gateway (PGW) 20 in a 4G network. Such PDN connection is a tunnel between the mobile terminal 10 and the PGW 20, i.e., the used access point (e.g., radio technology) is transparent for this connection. It consists of a default bearer, here the signaling bearer with a Quality of Service Class Identifier of QCI=5, and potentially other Evolved Packet System (EPS) bearers like the shown Dedicated Voice Bearer with QCI=1. Generally, an EPS bearer is a virtual connectivity between the mobile terminal 10 and the PGW 20. It can be for general usage (e.g. as a default bearer or for dedicated signaling and/or media usage). Dedicated bearers can be dynamically established by the communication network and may support different characteristics like QoS (signaling, voice, video, best effort etc.) and use so-called Traffic Flow Templates (TFT) as generally known in the art. In the IMS PDN connection as defined in GSMA PRD IR.92 (VoLTE), the signaling bearer is the default bearer, and dynamically established bearers may be used for SIP-controlled media like voice and/or video. Usually, the mobile terminal 10 has assigned one IP address per PDN connection, and the PDN connection is not changed when the access point (e.g., in terms of Radio Access Technology (RAT)) is changed.
In this scenario, if the IMS Access Point Name (APN) as defined by GSMA is used for both VoLTE and RCS, and a mobile terminal may change its network access between 3G and 4G networks, the following and other problems may occur.
Many currently deployed 3G/WCDMA networks do not support secondary Packet Data Protocol (PDP) contexts and thus cannot provide Network-Initiated Dedicated Bearers (NIDBs). Further, they may have limited support for parallel Packet Switched (PS) Radio Access Bearers (RABs). This may particularly occur in case the mobile terminal is roaming to another operator's network. As a consequence, in many cases all traffic on the IMS APN on 3G/WCDMA must be on a single bearer, namely the default bearer. Note that if a 3G network supports PRD IR.58 (VoHSPA), then there must be also secondary PDP context support and in this case NIDB can be handed over from 4G to 3G and vice versa. However, at present there are no networks supporting PRD IR.58.
At the same time, 4G/LTE access supports a default bearer (used as signaling bearer) and dynamically established dedicated media bearers. Thus, if a mobile terminal experiences a handover to or from 3G/WCDMA, there is the risk that media may be dropped (in the case of handover from 4G/LTE to 3G/WCDMA) or that the default bearer at QCI=5 will be overloaded (in the case of handover from 3G/WCDMA to 4G/LTE).
Accordingly, there is a need for a solution to mitigate these and other problems.